Problem Solution
Figure 101 The SDLC Frame Format
In bytes 1 1 or 2 1 or 2 Variable 2 1 in bytes 1 1 or 2 1 or 2 Variable 2 1 As Figure 10-1 shows, SDLC frames are bounded by a unique flag pattern. The address field always contains the address of the secondary involved in the current communication. Because the primary is either the communication source or destination, there is no need to include the address of the primary it is already known by all secondaries. The control field uses three different formats, depending on the type of SDLC frame...
OSPF Routers Not Establishing Neighbors
Symptom OSPF routers are not establishing neighbor relationships properly. The result is that routing information is not exchanged between routers. Table 7-11 outlines the problems that might cause this symptom and describes solutions to those problems. Table 7-11 OSPF Routers Not Establishing Neighbors Misconfigured or missing network router configuration command Step 1 Use the show ip ospf interfaces exec command to determine which interfaces have OSPF1 enabled. Step 2 If the output indicates...
Virtual Channel Connections and Virtual Paths
A virtual channel connection (VCC) is a series of virtual channel links (VCLs) between two ATM points. A VCL is a means of bidirectional transport of ATM cells between a point where a virtual channel identifier (VCI) value is assigned and the point where the same value is either reassigned or terminated. The VCI identifies the VCL to which a cell belongs and determines where the cell should go next. Figure 21-6 shows the relationship between VCLs and VCCs in an ATM network. Figure 21-6 The...
Figure 219 The User Network Interface ATM Cell Header Format
Table 21-2 Fields in an ATM Cell Header Table 21-2 Fields in an ATM Cell Header Controls the flow of traffic across the user network interface and thus into the ATM network. Second 4 bits of Byte 1 and the first 4 bits of Byte 2 Identifies a particular VPC3. A VPC is a group of virtual connections carried between two points and may involve several ATM links. VPIs provide a way to bundle traffic heading to the same destination. Second 4 bits of Byte 2, Byte 3, and the first 4 bits of Byte 4...
Cisco 4000 and Cisco 3000 Series Startup
When you start up a Cisco 4000 or a Cisco 3000 series router, the following should occur The System OK LED should come on and stay on as long as power is supplied. The fans should be operating. When the system has initialized successfully, the system banner should be displayed on the console screen. The system banner should look similar to the following System Bootstrap, Version 4.14(9), SOFTWARE Copyright (c) 1986-1994 by cisco Systems 4000 processor with 16384 Kbytes of main memory Loading...
ISDN Components
ISDN components include terminals, terminal adapters (TAs), network-termination devices, line-termination equipment, and exchange-termination equipment. ISDN terminals come in two types. Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals such as DTE that predate the ISDN standards are referred to as terminal equipment type 2 (TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN network...
Media Problems Token Ring
Table 6-1 outlines problems commonly encountered on Token Ring networks and offers general guidelines for solving those problems. Table 6-1 Media Problems Token Ring Table 6-1 Media Problems Token Ring Use the show interfaces token command to determine the status of the router's If the status line indicates that the interface and line protocol are not up, check the cable from the router to the MAU.1 Make sure that the cable is in good condition. If it is not, replace it. If you are performing a...
Figure 2110 ATM Adaptation Layer Functions
When ATM cells are transferred through a network, each cell is processed in isolation from all other cells. All processing decisions are made based on the cell header no processing of the data in the payload field occurs. Figure 21-11 shows some examples of AAL processing. Figure 21-11 AAL Processing Examples Figure 21-11 AAL Processing Examples Hosts A and C are connected to the network through ATM interfaces, so they do all their AAL processing internally. The network does not do any...
Split Horizon Example
Sometimes in a multipoint WAN environment it is desirable to leave split horizon disabled. However, steps should be taken to prevent routing information from being learned from the wrong interface or protocol. For example, in the environment shown in Figure 7-11, Router 2 might incorrectly receive information about RIP networks from Router 3 if the routers are not configured correctly. Figure 7-11 A Split Horizon Sample Network Figure 7-11 A Split Horizon Sample Network RIP routing information...
The show version Command
The show version command displays the image type, version number, and function sets that identify the exact software that is running on your router. Also displayed is the current configuration register setting. The following example displays sample output from the show version command Cisco Internetwork Operating System Software IOS (tm) GS Software (GS7-K-M), Version 11.0(9), RELEASE SOFTWARE (fcl) Copyright (c) 1986-1996 by cisco Systems, Inc. Compiled Tue 11-Jun-96 03 52 by tstevens Image...
Table 152 Serial Lines Increasing Output Drops on Serial Link
Input rate to serial interface exceeds bandwidth available on serial link Minimize periodic broadcast traffic such as routing and SAP1 updates by using access lists or by other means. For example, to increase the delay between SAP updates, use the ipx sap-interval interface configuration command. Increase the output hold queue size in small increments (for instance, 25 percent), using the hold-queue out interface configuration command. On affected interfaces, turn off fast switching for heavily...
Host and Router Subnet Mask Mismatch Example
In classful IP networks, every router and host in the same major network should share a common subnet mask. If there are disagreements on the length of the subnet mask, packets are not routed correctly. Table 7-10 shows how a UNIX host and a router will interpret an IP address differently if they have different subnet masks specified for the same major network. Table 7-10 Host and Router Subnet Mask Mismatch Example Table 7-10 Host and Router Subnet Mask Mismatch Example The host interprets the...
TCPIP Problems Forwarding BOOTP and Other UDP Broadcasts
Symptom Problems occur when forwarding BOOTP or other UDP broadcast packets. UDP broadcasts sent from network hosts are not forwarded by routers. Diskless workstations cannot boot. Table 7-7 outlines the problems that might cause this symptom and describes solutions to those problems. Table 7-7 TCP IP Problems Forwarding BOOTP and Other UDP Broadcasts TCP IP Problems Forwarding BOOTP and Other UDP Broadcasts Missing or misconfigured ip helper-address specification UDP broadcasts being forwarded...
Password Recovery Procedure Platforms Running Current Cisco IOS Releases
More recent Cisco platforms run from Flash memory or are netbooted and can ignore the contents of nonvolatile RAM (NVRAM) upon booting. (Cisco 7000 series routers that boot from Flash memory or netboot have this capability as well a Cisco 7000 that boots from ROM has this capability if it is running Cisco IOS Release 10.0 or later.) Ignoring the contents of NVRAM permits you to bypass the configuration file (which contains the passwords) and gain complete access to the router. You can then...
General Problem Solving Model
When you're troubleshooting a network environment, a systematic approach works best. Define the specific symptoms, identify all potential problems that could be causing the symptoms, and then systematically eliminate each potential problem (from most likely to least likely) until the symptoms disappear. Figure 1-1 illustrates the process flow for the general problem-solving model. This process flow is not a rigid outline for troubleshooting an internetwork it is a foundation from which you can...
Figure 82 IPX Packet Format
The fields of the IPX packet are as follows Checksum A 16-bit field that is set to ones. Packet length A 16-bit field that specifies the length, in bytes, of the complete IPX datagram. IPX packets can be any length up to the media maximum transmission unit (MTU) size. There is no packet fragmentation. Transport control An 8-bit field that indicates the number of routers the packet has passed through. When the value of this field reaches 15, the packet is discarded under the assumption that a...
Performing Loopback Tests
Loopback tests can help you pinpoint faults by looping a signal at various points in the network. The LightStream 2020 ATM switch provides the following two types of loopback tests Remote loopback test The remote loopback test loops data from an external device through the I O module and back. This test verifies that the data sent from the remote end can cross the telephone company line or cable, pass through the I O module, and return to the remote end. Internal loopback test The internal...
The Network Layer
The XNS network-layer protocol is called the Internet Datagram Protocol (IDP). IDP performs standard Layer 3 functions, including logical addressing and end-to-end datagram delivery across an internetwork. The format of an IDP packet is shown in Figure 14-1. A - Checksum B - Length C - Transport control D - Packet type E - Destination network number F - Destination host number G - Destination socket number H - Source network number I - Source host number J - Source socket number The fields of...
Cisco 7500 Series Startup
When you start up a Cisco 7500 series router, the following should occur The AC (or DC) OK LED should go on immediately and should remain on as long as the system is receiving power. The blower should be operating. The Route Switch Processor (RSP) and front-panel Normal LEDs should go on (to indicate normal system operation) and should remain on during system operation the CPU Halt LED should remain off. The Enabled LED on each interface processor should go on (to indicate that the RSP has...
Show interfaces ethernet Field Descriptions
Ethernet is up is administratively down Indicates whether the interface hardware is currently active and whether it has been taken down by an administrator. Disabled indicates that the router has received more than 5,000 errors in a keepalive interval, which is 10 seconds by default. is up down administratively down Indicates whether the software processes that handle the line protocol believe the interface is usable (that is, whether keepalives are successful) or if it has been taken down by...
Password Recovery Procedure IGS Running Software Prior to Software Release
Cisco IGS routers have a bank of DIP switches located on the rear panel. These DIP switches are used to set the hardware configuration register and must be used in the password-recovery process if the router is running system software prior to Software Release 9.1. Note If your password is encrypted, you cannot recover it. You must configure a new password. Figure 24-3 shows the password-recovery procedure for the Cisco IGS running software prior to Software Release 9.1. There is another...
Modem State Modem Hardware State Meaning
Idle CTS noDSR DTR RTS These are the proper modem states for connections between an access server or router and a modem (when there is no incoming call). Output of any other kind generally indicates a problem. Modem State Modem Hardware State Meaning Ready If the modem state is Ready instead of Idle, there are three Modem control is not configured on the access server or router. Configure the access server or router with the modem inout line configuration command. A session exists on the line....
Table 813 Novell IPX Slow Performance
Novell servers not processing SAP updates as quickly as router is generating them Step 1 Use the show interfaces privileged exec command to check for output drops. Step 2 If there are excessive drops, use the show ipx servers exec command on the router. Compare the output of this command with the output of the display servers system console command on Novell servers. Step 3 If the display servers output for a Novell server shows only a partial listing of the SAP entries shown by the router, the...
VINES Client on Serverless Network Cannot Connect to Server over PSN
Symptom Clients on a serverless network (that is, a network segment that has no attached VINES servers) cannot open a connection to a VINES server over a PSN. Table 13-3 outlines the problems that might cause this symptom and describes solutions to those problems. Table 13-3 VINES Client on Serverless Network Cannot Connect to Server over PSN VINES Client on Serverless Network Cannot Connect to Server over PSN Use the show running-config privileged exec command to view the For X.25...